In some paper pulping processes, a solution referred to as “oxidized white liquor” is used. Oxidized white liquor is typically made by oxidizing reducing compounds found in white liquor such as sodium sulfide, sodium polysulfide and sodium thiosulfate to form an oxidized white liquor having non-reducing compounds such as sodium sulfate therein.
A stirred tank of white liquor and either air or oxygen or a combination thereof and an external heat source is a common method of commercially producing white liquor as disclosed in U.S. Pat. Nos. 5,500,085 and 5,382,322.
Some paper mills, for example, those employing the kraft process, employ one or more oxygen delignification systems for removing lignin from wood chips and subsequently processed paper pulp. Lignin is a complex polymer in wood that binds to cellulose and forms a significant weight of the wood. The presence of lignin is detrimental to paper making where bright white paper is desired. Newspapers that turn yellow when left in the sun change color due to the presence of lignin in the paper. In the kraft process, some lignin may be removed in an initial digestion stage of wood chips carried out at high temperatures with the addition of liquids such as white liquor. The paper pulp which is obtained after further processing may be subjected to a tower oxygen delignification system upstream of a bleaching process. Various chemicals or substances may be used in the bleaching process.
One process of oxygen delignification in modern paper mills employs a basic liquid compound (e.g., alkali), pumped paper pulp fluid and oxygen gas under pressure. The combined material moves up a tall, vertically extending hollow tower and an oxygen delignification reaction occurs. The paper pulp fluid may be washed or not but travels down along conduit toward the ground. From here it may be subsequently treated (e.g., in a bleaching operation) or passed to an optional second hollow tower where secondary delignification occurs. The paper pulp fluid that has undergone secondary delignification may then travel along a conduit toward the ground and be further treated (e.g., in a bleaching operation). Very large amounts of fluid may be contained in the tower or towers, for example, two hundred thousand gallons. The oxygen delignification reaction that occurs inside the tower is slow. The delignification occurring in a single tower may take about 90 minutes, for example. During the oxygen delignification, due to the action of oxygen radicals on the lignin the bonds between the lignin and the cellulose of the paper pulp are broken. A goal of oxygen delignification in this process is to achieve a certain reduction in kappa number, for example, a reduction of at least 50%. This, along with subsequent bleaching, results in paper having suitable brightness. The oxygen delignification process is advantageous because it avoids the need to use certain chemicals for further bleaching and/or reduces the amounts of chemicals needed for bleaching. Reducing bleaching chemicals reduces cost and protects the environment.
The capital cost, operational costs and maintenance costs of the oxygen delignification tower system are very high. Such a tower costs many millions of dollars to construct. Paper mills could benefit from an improved oxygen delignification process, which uses equipment at a fraction of the capital cost, operates much faster, and achieves at least the same results as conventional tower oxygen delignification systems. It would be advantageous if such an improved system can be coupled with the conventional tower oxygen delignification system or can replace it entirely.